On cylinder bore surfaces of cylinder blocks, spray coatings for improving the corrosion resistance, wear resistance, and the like of the bore surfaces are formed by thermal spraying techniques in which a combustion flame is generated by an arc, plasma, gas, or the like, various metals or alloys are instantaneously melted, and spray particles which have been atomized (micro-particulated) by compressed air are made to impact/solidify and adhere. Such thermal spraying is performed by moving a spray gun in a rotating posture between both ends of a cylinder bore as disclosed in Patent Document 1, for example. However, when the spray particles impact the bore surface, spattering of the spray particles occurs, which causes such problems as the adhesion of these reflected particles and spray fumes to the bore surface.
This adhesion of the reflected particles and spray fumes is such that when the spray gun is moved from the upper end of the bore (the end portion of the cylinder bore that faces the outside of the cylinder block) to the lower end of the bore (the end portion of the cylinder bore that faces the inside of the cylinder block), they adhere to an unsprayed surface of the bore surface on the lower side. When the spray gun is moved from the lower end of the bore to the upper end, they adhere to an unsprayed surface of the bore surface on the upper side. As a result of the adhesion of the reflected particles and spray fumes to an unsprayed surface of the bore surface, there arise problems where the bond strength of the spray particles at the unsprayed surface drops significantly, and the smoothness of the bore surface is compromised.
As techniques for preventing the adhesion of the above-mentioned reflected particles and the like to the bore surface, the thermal spraying methods and thermal spraying apparatus disclosed in Patent Documents 2 and 3 may be cited. The thermal spraying method and thermal spraying apparatus of Patent Document 2 are an apparatus whose object is to prevent reducing operating efficiency resulting from the attachment/detachment of a masking member in a conventional thermal spraying apparatus that uses a masking member in order to prevent the adhesion of spray particles to parts where coating is unneeded. Specifically, they are such that spraying is performed while moving a spray gun in the axial direction within a cylinder in a state where the spray direction of the spray material from the spray gun is so inclined that the spray angle formed between this spray direction and the inner surface of the cylinder is an acute angle. The spray angle in the vicinity of the end portions of the inner surface of the cylinder in the axial direction is made greater than the spray angle at other regions.
In addition, the thermal spraying method of Patent Document 3 is one in which, at the lower half portion of the outer circumferential surface from a lower opening portion of a rotating hollow cylindrical tube, compressed air is blown against a recessed portion that is circumferentially provided in the shape of a ring in a plane that is parallel to the plane of the opening end, thereby causing an air flow that ascends along the inner circumferential surface, and removing unmelted particles and fumes adhered to the inner circumferential surface.    Patent Document 1    Japanese Patent Publication (Kokai) No. 7-62518 A (1995)    Patent Document 2    Japanese Patent Publication (Kokai) No. 2005-154802 A    Patent Document 3    Japanese Patent Publication (Kokai) No. 11-1758 A (1999)